The translational goal of Project 4 is to identify markers of the DNA damage response (DDR) pathway that will improve our ability to predict outcome in breast cancer and prevent the over, or under, treatinent of disease. Our foundation for evaluating candidate markers of outcome is a population-based cohort of 2337 women (approximately 1900 with available tumors) ages 45-79 diagnosed with invasive breast cancer who are being followed for recurrence and death (QUILT Study). This well-characterized cohort, which was specifically designed to assess determinants of recurrence and mortality, offers unique benefits in elucidating insights into outcomes, including: comprehensive pre- and post-diagnostic exposure data, complete treatment and medical history data, flexibility and efficiency in examining different hypotheses, information on and ability to control for many different potential confounders, and inclusion of a broad spectrum of cases. Breast cancer morbidity and mortality is substantial and there is an acute need for additional tumor markers (beyond histopathology, ER and Her2) that can predict outcome, serve as therapeutic targets, and/or guide therapy in newly diagnosed patients. Compelling evidence, largely centered on p53, indicates that the DDR pathway is a promising (but understudied) source of clinical prognostic and predictive markers for breast cancer. The lack of a clinically tractable assay to assess DDR activity has inhibited investigations to date and also, because DDR involves both p53-dependent and p53-independent responses, p53 status alone is an insufficient measure of activity or functionality of the signal transduction cascade. We will conduct a comprehensive assessment of the DDR pathway activity in human breast cancers via a multi-analyte marker panel designed to capture DDR pathway function. We will assess the association of DDR activity with breast cancer prognosis and treatment response, using the population-based cohort described above. Our specific aims are: (1) Using the QUILT Study population-based cohort and DDR markers identified from the literature and from discovery in Aim 2, test whether activation of the DNA damage response is predictive or prognostic in breast cancer; (2) Identify proteins and transcripts elevated upon activation of the DNA damage response in human mammary epithelial cells (ex vivo), and determine which of the responsive proteins are detectable in human breast cancer tissues.